In recent years, a movement has gained traction to replace incandescent light bulbs with lighting fixtures that employ more efficient lighting technologies as well as to replace relatively efficient fluorescent lighting fixtures with lighting technologies that produce a more pleasing, natural light. One such technology that shows tremendous promise employs light emitting diodes (LEDs). Compared with incandescent bulbs, LED-based light fixtures are much more efficient at converting electrical energy into light, are longer lasting, and are also capable of producing light that is very natural. Compared with fluorescent lighting, LED-based lighting fixtures are also very efficient, but are capable of producing light that is much more natural and more capable of accurately rendering colors. As a result, lighting fixtures that employ LED technologies are replacing incandescent and fluorescent bulbs in residential, commercial, and industrial applications.
Unlike incandescent bulbs that operate by subjecting a filament to a desired current, LED-based lighting fixtures require electronics to drive one or more LEDs. The electronics generally include a power supply and special control circuitry to provide uniquely configured drive signals that are required to drive the one or more LEDs in a desired fashion. Typically, the drive signal is a pulse width modulated (PWM) signal, which has a fixed period and a variable duty cycle. Varying the duty cycle varies of the light output of the LED-based lighting fixtures. The greater the duty cycle, the higher the light output, and vice versa.
During each period of the PWM signal, there is an active portion where current flows through the LEDs and an inactive portion where current does not flow through the LEDs. As a result, the LEDs rapidly turn on and off during each period of the PWM signal. The frequency of the PWM signal, which is equal to the inverse of the period of the PWM signal, is kept high enough so that rapidly turning the LEDs on and off is imperceptible to the human eye. Unfortunately, digital image capture systems, such as still and video cameras and the like are not as forgiving. Capturing images in an environment where LED-based lighting fixtures use PWM signals causes various undesirable artifacts, including banding. Banding is the phenomena where dark horizontal bands extend across the entire image and are distributed vertically throughout the entire image.
Banding is caused when portions of a video frame are being captured when the PWM signal is active and the LED-based lighting fixture is outputting light, and other portions of the video frame are being captured when the PWM signal is inactive and the LED-based lighting fixture is not outputting light. The dark bands correspond to the portions of the image that are captured when the PWM signal is inactive and the LED-based lighting fixture is not outputting light. The lighter bands that separate the dark bands correspond to the portions of the frame that are captured when the PWM drive signal is active and the LED-based lighting fixture is outputting light. The dark bands may be stationary or may move in a continuous fashion up or down within the captured video. The thickness, spacing, and darkness of the dark bands depend on the interaction between integration times and frame rates of the video camera and the frequency and duty cycle associated with the PWM signal used by the LED-based lighting fixtures.
Banding detracts from the user's visual experience and gives the perception that the associated equipment is of lesser quality or malfunctioning. Banding can also cause issues with various video processing, such as encoding and compression, which is required for virtually all video.
With a global shutter camera, the unwanted effects associated with the frame rate and/or integration time of the camera interacting or intermodulating with the PWM frequency of the light source can also manifest themselves as the screen decreasing then increasing in brightness along successive frames or having undesired levels of illumination in a still frame.
Other adverse effects are possible when image processing rates, such as the integration rate time and/or frame rate of the camera or image sensor negatively interact with frequency components of the LED light source, such as the PWM frequency, that results in periodic changes in light intensity or color.
Accordingly, there is a need for techniques to improve LED-based lighting fixtures, image capture systems, and a combination thereof to address the adverse intermodulation effects of the image processing rates of the camera or image sensor and the frequency components in the LED light sources, for example the banding or changes in brightness issues brought on by the LED-based lighting fixtures that employ PWM signals.